Optical film and liquid-crystal display device

ABSTRACT

An optical film has a transparent film and an adhesive layer provided on one surface of the transparent film. The adhesive layer has a refractive index different by 0.12 or less from that of a layer of the one surface of the transparent film. The transparent film has an average in-plane retardation of not larger than 30 nm. A repetitive prismatic structure is provided on the other surface of the transparent film, the repetitive prismatic structure having optical path changing slopes aligned in a substantially constant direction at an inclination angle in a range from 35 to 48 degrees with respect to a plane of the transparent film.

This is a divisional of application Ser. No. 09/758,165 filed Jan. 12,2001, now U.S. Pat. No. 6,747,801, the disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical film by which the opticalpath of the light incident on one of side surfaces of a liquid-crystaldisplay device is changed to a viewing direction efficiently, and withwhich it is possible to form a transmission type orreflection-transmission double type liquid-crystal display device whichis small in thickness, which is light in weight, which is excellent inbrightness and uniformity of brightness and display of which is easy toview.

The present application is based on Japanese Patent Applications No.2000-4241, 2000-21309 and 2000-85718, which are incorporated herein byreference.

2. Description of the Related Art

Greater reduction in thickness, size and weight of transmission typeliquid-crystal display devices has been demanded for purposes ofsuppression of increase in weight which is accompanied by increase insize of television and personal computer display screens, reduction insize and weight of portable personal computers and portable telephonesets, etc. In the meanwhile, it is difficult to reduce thickness, sizeand weight of a transmission type liquid-crystal display device providedwith a back-lighting system using a background-art bottom type orside-lighting type light pipe. Incidentally, the bottom-typeback-lighting system generally has a thickness of not smaller than 4 mmbecause an illuminator, a light diffusing plate and a reflector aredisposed just under a liquid-crystal display panel. Even theside-lighting type light pipe has a thickness of not smaller than 1 mmunder the necessity of light transmission. When a light diffusing plate,a reflector, a prism sheet, etc. are disposed on the side-lighting typelight pipe, the total thickness generally reaches a value of not smallerthan 3 mm.

A liquid-crystal display device in which a half-transmission typereflector is disposed between the aforementioned transmission typeliquid-crystal display panel and a back-lighting system is heretoforeknown as a reflection-transmission double type liquid-crystal displaydevice which can be viewed in a reflection mode by using external light.The half-transmission type reflector is disposed in order to makeviewing in a reflection mode possible. If there is no half-transmissiontype reflector, viewing in a reflection mode by using external light isso dark that the liquid-crystal display device substantially hardlyfunctions as a reflection type liquid-crystal display device. Theaddition of the half-transmission type reflector, however, makes thevolume and weight of the liquid-crystal display device larger. Moreover,light is diverged into transmitted light and reflected light by thehalf-transmission type reflector. There is therefore a problem that notonly viewing in a transmission mode but also viewing in a reflectionmode becomes dark so that brightness in a reflection mode is inferior tothat of a reflection exclusive type liquid-crystal display device usinga high-reflectance reflection layer.

SUMMARY OF THE INVENTION

An object of the present invention is to develop an optical film bywhich the optical path of the light incident on one of side surfaces ofa liquid-crystal display device is changed to a viewing directionefficiently, and with which it is possible to form a transmission typeor reflection-transmission double type liquid-crystal display devicewhich is small in thickness, which is light in weight, and display ofwhich is bright and easy to view.

According to a first aspect of the present invention, there is providedan optical film comprising: a transparent film; an adhesive layerprovided on one surface of the transparent film, the adhesive layerhaving a refractive index different by 0.1 or less from a refractiveindex of a layer of the one surface of the transparent film; and arepetitive prismatic structure provided on the other surface of thetransparent film, the repetitive prismatic structure having optical pathchanging slopes aligned in a substantially constant direction at aninclination angle in a range of from 35 to 48 degrees with respect to aplane of the transparent film.

According to a second aspect of the present invention, there is alsoprovided an optical film comprising: a transparent film having anaverage in-plane retardation of not larger than 30 nm; an adhesive layerprovided on one surface of the transparent film, the adhesive layerhaving a refractive index different by 0.12 or less from a refractiveindex of a layer of the one surface of the transparent film; and arepetitive prismatic structure provided on the other surface of thetransparent film, the repetitive prismatic structure having optical pathchanging slopes aligned in a substantially constant direction at aninclination angle in a range of from 35 to 48 degrees with respect to aplane of the transparent film.

According to a third aspect of the present invention, there is alsoprovided an optical film comprising: a transparent film having arefractive index of not lower than 1.49; transparent adhesive meansprovided on one surface of the transparent film, the transparentadhesive means having a refractive index of not lower than 1.49; and arepetitive prismatic structure provided on the other surface of thetransparent film, the repetitive prismatic structure of having opticalpath changing slopes aligned in a substantially constant direction at aninclination angle in a range of from 35 to 48 degrees with respect to aplane of the transparent film. There is further provided aliquid-crystal display using the optical film.

The optical film according to the present invention is disposed alongthe viewing surface of a liquid-crystal display panel having anilluminator on one of side surfaces of the panel. Hence, the opticalpath of the light incident on the side surface or the transmission lightof the incident light is changed efficiently to the viewing direction ofthe liquid-crystal display panel by optical path changing slopesdisposed on the optical film. Hence, the light can be utilized forliquid-crystal display in a transmission mode. Hence, it is possible toform a transmission type liquid-crystal display device which is small inthickness and light in weight, which is excellent in brightness anduniformity of brightness, which is low in display unevenness and whichis excellent in display quality. Moreover, because flat surface portionsare disposed between the optical path changing slopes in the opticalfilm, external light can be made to enter efficiently through the flatsurface portions. Hence, when the entering external light is reflectedby the reflection layer, the external light can be utilized forliquid-crystal display in a reflection mode. A reflection mode systemcan be formed as well as the aforementioned transmission mode system.Hence, it is possible to form a transmission-reflection double typeliquid-crystal display device which is small in thickness and light inweight, which is excellent in brightness and uniformity of brightness,which is low in display unevenness and which is excellent in displayquality.

The aforementioned effect is produced by an optical path control typeoptical film which mainly uses slope reflection to control the opticalpath of light. That is, the light incident on one of side surfaces ofthe liquid-crystal display panel or the transmission light of theincident light is reflected by optical path changing slopes so that theoptical path of the light can be changed with good directivity. Hence,good visibility in a transmission mode can be achieved. Moreover, flatsurfaces can be disposed easily between the optical path changingslopes. Hence, external light is transmitted through the flat surfacesso that entering of external light can be ensured sufficient. Hence,good visibility in a reflection mode can be also achieved. In a methodof scatter reflection by a roughened surface of a scattering sheet 6shown in FIG. 14, or the like, it is difficult to achieve theaforementioned effect. Incidentally, JP-A-5-158033 discloses areflection type liquid-crystal display device in which illuminationlight is made incident on one of side surfaces of a liquid-crystaldisplay panel and totally reflected by a visual side cell substrate andin which the reflected light is scattered by a roughened surface typereflector so that the scattered light is utilized for display.

In the aforementioned case, however, light allowed to be utilized fordisplay is that which exits from the panel due to coming contrary to thetotal reflection condition by scattering. Generally, scattered lightexhibits a normal distribution having a direction of regular reflectionas a peak, in Extended Abstracts (the 20th Liquid-Crystal DiscussionLecture) 3 G510, Tohoku University; Uchida et al. Hence, theaforementioned display light is the light hardly utilized efficientlyfor display and greatly inclined with respect to a frontal (vertical)direction. Hence, the display becomes dark in the frontal direction.Nevertheless, intensifying scattering by the roughened surface typereflector is unfavorable for display in consideration of viewing in areflection mode because the quantity of light in the frontal directionin the reflection mode is reduced (SID 96 DIGEST pp. 149–152). In theroughened surface scatter reflection method, it is, therefore, difficultto obtain scattering intensity favorable to the two modes becausescattering intensity required of the transmission mode is antinomic toscattering intensity required of the reflection mode.

On the other hand, according to the present invention, the optical pathcontrol type optical film, which uses slope reflection to control theoptical path of light, mainly utilizes light exhibiting a peak in adirection of regular reflection and controls the optical path of thereflected light. Hence, directivity, especially frontal directivity,favorable for display can be provided easily. Hence, a brighttransmission mode can be achieved. Also in a reflection mode, flatportions of the optical film except the optical path changing slopes canbe utilized, and efficient entrance, reflection and transmission ofexternal light can be ensured. Hence, the state of light can be balancedeasily so as to be favorable to both reflection and transmission modes.Further, according to the present invention, the optical film isdesigned to be able to be bonded to a glass substrate, or the like, of aliquid-crystal cell through adhesive means having a large refractiveindex. Hence, as indicated by the arrows in FIG. 7, the total reflectionon the bonding interfaces can be reduced in the optical film to therebymake it possible to form a transmission type or reflection-transmissiondouble type liquid-crystal display device which is excellent inbrightness and uniformity of brightness, which is low in displayunevenness and which is excellent in display quality. If the totalreflection were great in the optical film, the quantity of light whichis incident on one of side surfaces of the cell and which is transmittedthrough the cell from the side surface so as to enter the optical filmwould be reduced. Particularly, transmission light which is moreapproximately parallel with the cell substrate as indicated by the arrowin FIG. 10 would cause total reflection more easily. That is, lightwhich is transmitted to a position farther from the incidence sidesurface would cause total reflection more easily. As a result,brightness in a position far from the incidence side surface would bereduced to increase variation in brightness. Hence, display quality ofthe display device would be lowered.

Features and advantages of the invention will be evident from thefollowing detailed description of the preferred embodiments described inconjunction with the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1H are explanatory side views showing examples of an opticalfilm (optical path changing slopes);

FIG. 2 is a plan view showing an example of optical path changingslopes;

FIG. 3 is a plan view showing another example of the optical pathchanging slopes;

FIG. 4 is a plan view showing a further example of the optical pathchanging slopes;

FIG. 5 is a side view showing another example of the optical film;

FIG. 6 is a side view showing a further example of the optical film;

FIG. 7 is a sectional view showing an example of a liquid-crystaldisplay device (an explanatory view of a relationship between arefractive index and an optical path);

FIG. 8 is a sectional view showing another example of the liquid-crystaldisplay device;

FIG. 9 is a sectional view showing a further example of theliquid-crystal display device;

FIG. 10 is a view of another relationship between a refractive index andan optical path;

FIG. 11 is a view showing a transmission state of polarized light;

FIG. 12 is a view showing another transmission state of polarized light;

FIG. 13 is a section view showing an example of a liquid-crystal displaydevice wherein light may be enclosed by the optical film due tointerference reflection between the transparent film and the tacky layerdue to the difference in refractive index therebetween.

FIG. 14 is a sectional showing an example of a background-arttransmission type liquid-crystal display device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The optical film according to the present invention comprises: atransparent film; an adhesive layer provided on one surface of thetransparent film, the adhesive layer having a refractive index differentby 0.1 or less from that of a layer of the one surface of thetransparent film; and a repetitive prismatic structure provided on theother surface of the transparent film, the repetitive prismaticstructure having optical path changing slopes aligned in a substantiallyconstant direction at an inclination angle in a range of from 35 to 48degrees with respect to a plane of the transparent film.

The optical film from another aspect of the present invention comprises:a transparent film having an average in-plane retardation of not largerthan 30 nm; an adhesive layer provided on one surface of the transparentfilm, the adhesive layer having a refractive index different by 0.12 orless from a refractive index of a layer of the one surface of thetransparent film; and a repetitive prismatic structure provided on theother surface of the transparent film, the repetitive prismaticstructure having optical path changing slopes aligned in a substantiallyconstant direction at an inclination angle in a range of from 35 to 48degrees with respect to a plane of the transparent film.

The optical film from still another aspect of the present inventioncomprises: a transparent film having a refractive index of not lowerthan 1.49; transparent adhesive means provided on one surface of thetransparent film, the transparent adhesive means having a refractiveindex of not lower than 1.49; and a repetitive prismatic structureprovided on the other surface of the transparent film, the repetitiveprismatic structure having optical path changing slopes aligned in asubstantially constant direction at an inclination angle in a range offrom 35 to 48 degrees with respect to a plane of the transparent film.

FIGS. 1A to 1H show examples of the optical film. The reference numeral1 designates an optical film; 11, a transparent film; 12, adhesivemeans; 13, a layer of a repetitive prismatic structure, that is, a layerof a repetitive structure of a plurality of optical path changing meansA having optical path changing slopes A1; and 14, a strip sheet. Therepetitive structure of the plurality of optical path changing means Amay be formed integrated with the transparent film 11, as illustrated inFIG. 1G.

As illustrated in FIG. 7, the optical film 1 is disposed along a viewingsurface of a liquid-crystal display panel P having an illuminator 5 onone of its side surfaces. That is, the light incident on the sidesurface from the illuminator 5 or the transmission light of the incidentlight is reflected by the optical path changing slopes A1 as indicatedby the arrow in FIG. 7. The optical path of the reflected light ischanged toward a non-slope-forming surface of the transparent film 11,that is, toward the viewing direction of the liquid-crystal displaypanel P, so that the light is made to exit from the transparent film 11.The optical film 1 is provided for the purpose of being capable ofutilizing the exit light as illumination light (display light) for theliquid-crystal display panel, etc.

The transparent film having a refractive index of not lower than 1.49can be formed of a suitable material exhibiting transparency andcorresponding to the wavelength range of light which is made to enterthe film from the illuminator, or the like. Incidentally, examples ofthe suitable material used in a visible light range include: transparentresin represented by acrylic resin, polycarbonate resin, celluloseresin, norbornene resin, or the like; curable resin which can bepolymerized by heat, by ultraviolet rays, or by radial rays such aselectron rays; and so on. From the point of view to enhance theefficiency of incidence of the light incident on the optical pathchanging slopes to thereby obtain a liquid-crystal display deviceexcellent in brightness and uniformity of brightness, the refractiveindex of the transparent film is preferably not lower than 1.50, morepreferably not lower than 1.51, further preferably not lower than 1.52.Although such a refractive index is generally based on a D line in thecase of the visible light range, it is not limited to the above valuesbut may be set in accordance with the wavelength range of the incidentlight if the incident light has peculiarity with respect to thewavelength range (the same rule is applied hereinafter).

From the point of view to restrain luminance unevenness or color shadingto obtain a liquid-crystal display device low in display unevenness, itis preferable that the transparent film exhibits no birefringence orsmall birefringence, and particularly has an average in-planeretardation of not larger than 30 nm. When the tranparent film is madeto have a small retardation, and linerarly polarized light entersthrough a polarizer, or the like, the polarized state of the light canbe kept satisfactory advantageously to prevention of the display qualityform being deteriorated.

That is, when the transparent film 11 having a small retardation asillustrated in FIG. 11 is used in the case where linearly polarizedlight enters the transparent film 11 through a polarizer 31, or thelike, the polarized state of the light can be kept excellent so that thedisplay quality can be prevented from being deteriorated. Incidentally,a polyester film, or the like, however, generally exhibits a retardationof about 2,000 nm. If the film having a large retardation as illustratedin FIG. 12 is used in the case where linearly polarized light enters thefilm through the polarizer 31, or the like, the light may be iridizeddue to a color shift in accordance with its incident angle or reflectionangle under the influence of the retardation, or the polarized state maybe changed to lower the transmission efficiency or exit efficiency ofthe light. As a result, display unevenness such as luminance unevennessor color shading is apt to appear undesirably. From the point of view toprevent display unevenness, the average in-plane retardation of thetransparent film is preferably not larger than 20 nm, more preferablynot larger than 15 nm, further preferably not larger than 10 nm, and itis also preferable that the retardation in the transparent film fromplace to place varies as small as possible. Further, if it is necessaryto restrain internal stress, which is easily produced in the transparentfilm in the bonding process, so as to prevent a retardation from beingproduced due to the internal stress, the transparent film is preferablymade of a material having a low photoelastic coefficient.

In addition, in consideration that the incidence angle of transmissionlight onto the transparent film is apt to exceed 45 degrees, the averagethicknesswise retardation of the transparent film is apt to have aninfluence on the incidence angle in the same manner as that in theaverage in-plane retardation. Therefore, the average thicknesswiseretardation is preferably not larger than 50 nm, more preferably notlarger than 30 nm, further preferably not larger than 20 nm, from thepoint of view to prevent display unevenness. The transparent film withsuch a small retardation may be formed by a suitable method, forexample, by a method in which internal optical strain is eliminated byan existing means for annealing a film. Casting is a preferable methodto form a transparent film with such a small retardation. Incidentally,the aforementioned retardation conditions concerning the transparentfilm does not have to be satisfied all over the whole surface of theoptical film. It will go well if the retardation conditions aresatisfied in a display-available extent of the optical film. Inaddition, it is preferable that the retardation is based on light in avisible range, particularly based on light of a wavelength of 550 nm.

To achieve the aforementioned purpose, the transparent film 11 isprovided with slopes A1 as shown in FIGS. 1A to 1H. The slopes A1 areprovided on one side of the transparent film 11, and reflect theincident light on one of side surfaces or the transmission light of theincident light to thereby change the optical path of the light. On thisoccasion, from the point of view of obtaining illumination lightexcellent in frontal directivity through optical path change, thetransparent film 11 is configured as shown in FIGS. 1A to 1H. That is,according to the present invention, the transparent film 11 is formed tohave a repetitive prismatic structure, that is, a repetitive structureof a plurality of optical path changing means A containing optical pathchanging slopes A1 aligned in an approximately constant direction so asto be inclined at an inclination angle θ1 in a range of from 35 to 48degrees with respect to the film plane.

FIGS. 1A to 1H show various examples of each of the plurality of opticalpath changing means A having optical path changing slopes A1. In FIGS.1A to 1C, and FIGS. 1G and 1H, each of the optical path changing means Ais substantially shaped like a triangle in section. In FIGS. 1D and 1E,each of the optical path changing means A is substantially shaped like atetragon in section. In FIG. 1F, each of the optical path changing meansA is substantially shaped like a pentagon in section. More specifically,in FIG. 1A, each of the optical path changing means A has two opticalpath changing slopes A1 and is shaped like an isosceles triangle insection. In FIGS. 1B, 1G and 1H, each of the optical path changing meansA has an optical path changing slope A1, and a steep slope A2 having aninclination angle larger than that of the slope A1. In FIG. 1C, theoptical path changing means A are provided as a repetitive structure ofoptical path changing means A each having a combination of an opticalpath changing slope A1 and a gentle slope A3 having an inclination anglesmaller than that of the slope A1. In FIG. 1C, the optical path changingmeans A are formed over the whole surface of one side of the transparentfilm 11 so that they are adjacently continued to one another. In FIGS.1A to 1C and FIGS. 1E, 1G and 1H, the plurality of optical path changingmeans A are constituted by concave portions (grooves). In FIGS. 1D and1F, the plurality of optical path changing means A are constituted byconvex portions (protrusions).

Hence, the optical path changing means may be formed from concave orconvex portions constituted by equal-side surfaces or slopes havingequal inclination angles as described above. Alternatively, the opticalpath changing means may be formed from concave or convex portionsconstituted by a combination of optical path changing slopes and steepor gentle slopes, or slopes different in inclination angle. The shape ofthe optical path changing means can be determined suitably correspondingto the number of incidence side surfaces and the position of eachincidence side surface on which the light is incident. From the point ofview of improving mar-proofness to keep the slope function high, aplurality of optical path changing means constituted by concave portionsare superior to a plurality of optical path changing means constitutedby convex portions because the slopes, etc., in the concave portions arehardly damaged.

The optical film preferable from the point of view of achieving theaforementioned characteristic such as frontal directivity has opticalpath changing slopes A1 which are aligned in a substantially constantdirection so as to face the incidence side surface on which the light isincident. Hence, when light is made incident on two or more sidesurfaces of the optical film 1, for example, as shown in FIG. 9, it ispreferable to use an optical film having optical path changing slopes A1corresponding to the number and positions of the incidence sidesurfaces.

Incidentally, when opposite two side surfaces of the optical film 1 areused as incidence side surfaces on which the light is incident as shownin FIG. 9, there is preferably used an optical film 1 constituted by aplurality of optical path changing means containing two or more kinds ofoptical path changing slopes. Among the two or more kinds of opticalpath changing slopes, one kind of optical path changing slopes alignedin an approximately constant direction serve as a reference whileanother kind of optical path changing slopes are aligned in a directionopposite to the reference optical path changing slope. Examples of theoptical film preferably used include: an optical film 1 constituted by aplurality of optical path changing means A each of which is shaped likean isosceles triangle in section by two optical path changing slopes A1as shown in FIG. 1A; and an optical film 1 constituted by a plurality ofoptical path changing means A each of which contains two optical pathchanging slopes A1 and each of which is substantially shaped like atrapezoid, a tetragon or a pentagon in section as shown in FIGS. 1D, 1Eand 1F so that the ridgelines each of which includes two opticalchanging slopes A1 are parallel to the incidence side surfacesrespectively.

When two adjacent cross side surfaces of the optical film are used asincidence side surfaces on which the light is incident, there ispreferably used an optical film having two kinds of optical pathchanging slopes A1 corresponding to the incidence side surfaces so thatthe ridgelines of the two kinds of optical path changing slopes A1 areparallel to the two cross side surfaces respectively. When three or moreside surfaces inclusive of opposite side surfaces and adjacent crossside surfaces are used as incidence side surfaces on which the light isincident, there is preferably used an optical film having optical pathchanging slopes A1 constituted by a combination of the aforementionedslopes.

As described above, the optical path changing slopes A1 play a role ofreflecting the light incident on the slopes A1, among the light incidenton the incidence side surface and the transmission light of the incidentlight, to thereby change the optical path of the light. In this case,when the inclination angle θ1 of each of the optical path changingslopes A1 with respect to the film plane is selected to be in a range offrom 35 to 48 degrees as illustrated in FIG. 1A, the optical path of thelight incident on the side surface or the transmission light of theincident light can be changed so as to be sufficiently perpendicular tothe film plane. Accordingly, illumination light excellent in frontaldirectivity can be obtained efficiently.

If the inclination angle θ1 is smaller than 35 degrees, the optical pathof the reflected light is largely shifted by 30 degrees or more from thefrontal direction. Accordingly, the reflected light is difficult to beutilized effectively for display, and frontal luminance may thereforerun short. On the other hand, if the inclination angle θ1 is larger than48 degrees, the condition for total reflection of the light incident onthe incidence side surface or the transmission light of the incidentlight cannot be satisfied. Accordingly, light leaking from the opticalpath changing slopes increases, and efficiency of utilization of thelight incident on the side surface may therefore run short. From thepoint of view of optical path change excellent in frontal directivity,suppression of leaking light, etc., and in consideration of thecondition for total reflection of the transmission light on the basis ofrefraction in Snell's law, the inclination angle θ1 of each of theoptical path changing slopes A1 is preferably in a range of from 38 to45 degrees, more preferably in a range of from 40 to 44 degrees.

The plurality of optical path changing means A having the optical pathchanging slopes A1 are formed as a repetitive prismatic structure forthe purpose of reducing the thickness of the optical film. In this case,it is necessary to reflect the light incident on the incidence sidesurface and transmit the reflected light toward the counter side surfaceefficiently so as to emit light on the whole surface of the optical filmas uniformly as possible. From this point of view, it is preferable thatthe optical path changing means A are formed as a structure includingflat surfaces which are constituted by gentle slopes A3 inclined at aninclination angle of not larger than 5 degrees, particularly not largerthan 4 degrees, more particularly not larger than 3 degrees with respectto the film plane, or which are constituted by film surfaces A4 inclinedat an inclination angle of about 0 degree with respect to the film planeas shown in FIGS. 1A to 1H. Therefore, the optical path changing means Aincluding steep slopes A2 as illustrated in FIGS. 1B, 1G and 1H arepreferably formed as a structure in which the angle of the steep slopesA2 is selected to be not smaller than 35 degrees, particularly notsmaller than 50 degrees, more particularly not smaller than 60 degreesso that the width of the film surfaces A4 can be enlarged.

When a reflection layer 4 is disposed on the back side of the opticalfilm 1 as illustrated in FIGS. 7 to 9, the flat surfaces constituted bygentle slopes A3 or film surfaces A4 can function as incidence portionson which external light is made incident and as transmission portionsthrough which the reflected light of the incident light by thereflection layer 4 is transmitted. Hence, display can be made in areflection mode (in an external light mode) by using external light in acondition that the illuminator is switched off. Hence, areflection-transmission double type liquid-crystal display device can beformed.

In the aforementioned case, particularly when the optical path changingmeans A are formed as a repetitive structure in which the optical pathchanging means A are adjacent to one another and each of the means A hasslopes A1 and A3 as shown in FIG. 1C, the angle difference betweeninclination angles of the gentle slopes A3 with respect to the filmplane is selected preferably to be not larger than 5 degrees, morepreferably not larger than 4 degrees, further preferably not larger than3 degrees on the whole of the optical film, and the angle differencebetween inclination angles of adjacent ones of the gentle slopes A3 isselected preferably to be not larger than 1 degree, more preferably notlarger than 0.3 degrees, further preferably not larger than 0.1 degrees.This angle difference selection is to prevent the optical path of thelight reflected by the gentle slopes A3 from changing largely,especially from changing largely in between adjacent ones of the gentleslopes A3. This rule is also applied to the plurality of optical pathchanging means A constituted by the slopes A1 and A3 as shown in FIG.1F.

From the point of view of obtaining bright display in an external lightmode, the projected area or width, on the film plane, of the flatsurfaces constituted by gentle slopes A3 or film surfaces A4 each havingan inclination angle of not larger than 5 degrees with respect to thefilm plane is selected preferably to be not smaller than 10 times, morepreferably not smaller than 12 times, further preferably not smallerthan 15 times as large as the projected area or width, on the filmplane, of the slopes A1 or A2 each having an inclination angle of notsmaller than 35 degrees with respect to the film plane on which theoptical path changing means A are formed. This projected area or widthselection is to improve efficiency of incidence of external light andefficiency of transmittance of the light reflected by the reflectionlayer.

From the point of view to restrain luminance unevenness or color shadingto obtain a liquid-crystal display device low in display unevenness, itis preferable that the transparent film exhibits no birefringence orsmall birefringence, and particularly has an average in-planeretardation of not larger than 30 nm. When the transparent film is madeto have a small retardation, and linearly polarized light enters througha polarizer, or the like, the polarized state of the light can be keptsatisfactory advantageously to prevention of the display quality frombeing deteriorated.

As illustrated in FIGS. 2 to 4, the plurality of optical path changingmeans A are provided so that the ridgelines of the optical path changingmeans A are parallel to or inclined to the incidence side surface onwhich light is incident. In this case, the optical path changing means Amay be formed so as to be continued from one end to the other end of theoptical film 1 as illustrated in FIGS. 2 and 3, or may be formedintermittently and discontinuously as illustrated in FIG. 4. When theplurality of optical path changing means A are formed discontinuously,it is preferable from the point of view of efficiency of incidence ofthe transmission light, efficiency of chancing the optical path, etc.that the length of each prismatic structures of a groove or a protrusionalong the direction of the incidence side surface is selected to be notsmaller than 5 times as large as the depth or height of the prismaticstructure. It is further preferable from the point of view of uniformlight emission on the optical film that the length is selected to be notlarger than 500 μm, particularly in a range of from 10 to 480 μm, moreparticularly in a range of from 50 to 450 μm. Moreover, it is preferablethat a projected area of the discontinuous grooves onto an area of thefilm plane is not larger than 10%.

Any suitable surface shape such as a linear surface, a bent surface, acurved surface, etc., maybe formed as the shape of each of the slopesfor constituting the optical path changing means A. The sectional shapeof the optical path changing means A and the repetition pitch of theoptical path changing slopes A1 are not particularly limited. They canbe determined suitably in accordance with the uniformity of lightemission on the optical film in a transmission (switching-on) modebecause the optical path changing slopes A1 are factors for determiningluminance in the transmission mode. They can be further determinedsuitably in accordance with the uniformity of light emission in anexternal light mode in a reflection-transmission double typeliquid-crystal display device. Hence, the quantity of light the opticalpath of which is changed can be controlled on the basis of thedistribution density of the slopes.

Therefore, the inclination angles of the slopes A1, A2, A3, etc., may beuniform on the whole surface of the film, or may vary so that theoptical path changing means A is enlarged as the location goes fartherfrom the incidence side surface on which the light is incident, asillustrated in FIG. 5, for the purpose of making light emission on theoptical film uniform against absorption loss and attenuation oftransmission light due to the optical path changing. The optical pathchanging means A may be disposed at regular intervals of a predeterminedpitch as illustrated in FIGS. 2 and 3. Alternatively, the optical pathchanging means A may be disposed at irregular intervals so that thepitch is shortened as the location goes farther from the incidence sidesurface on which the light is incident. Accordingly, the distributiondensity of the optical path changing means A is made gradually higher,as illustrated in FIGS. 4 and 6. Alternatively, the optical pathchanging means may be disposed at a random pitch so that light emissionon the optical film can be made uniform. The random pitch is favorableto prevention of moire caused by interference with pixels. Therefore,the optical path changing means A may be constituted by a combination ofprismatic structures different in shape, or the like, as well as pitch.Incidentally, in FIGS. 2 to 6, the arrow shows the direction oftransmission of the light incident on the incidence side surface.

When a reflection-transmission double type liquid-crystal display deviceis provided, unnatural display may be caused by shortage of transmissionof display light if the optical path changing slopes A1 overlap pixelsof the liquid-crystal display panel. From the point of view ofpreventing the unnatural display from occurring, etc., it is preferablethat the overlap area between the pixels and the slopes A1 is reduced asmuch as possible to thereby ensure sufficient light transmittancethrough the flat surfaces A3 or A4. From this point of view and inconsideration that the pixel pitch of the liquid-crystal display panelis generally in a range of from 100 to 300 μm, each of the optical pathchanging slopes A1 is selected preferably to be not larger than 40 μm,more preferably in a range of from 3 to 20 μm, further preferably in arange of from 5 to 15 μm in terms of the projected width on the filmplane. The projected width is also preferable from the point of view ofpreventing display quality from being lowered because of diffraction inconsideration that the coherent length of a fluorescent tube isgenerally about 20 μm.

It is preferable from the aforementioned point of view that the distancebetween adjacent ones of the optical path changing slopes A1 is large.As described above, however, the optical path changing slopes serve as afunctional portion for substantially generating illumination light bychanging the optical path of the light incident on the side surface.Hence, if the distance is too large, illumination becomes sparse in aswitched-on mode so that display may be unnatural. In consideration ofthese facts, the repetition pitch of the optical path changing slopes A1is preferably selected to be not larger than 5 mm, more preferably in arange of from 20 μm to 3 mm, further preferably in a range of from 50 μmto 2 mm.

When the optical path changing means are constituted by a repetitiveprismatic structure, moire may occur because of interference between theoptical path changing means and the pixels of the liquid-crystal displaypanel. Although moire can be prevented by adjustment of the pitch in therepetitive prismatic structure, the pitch in the repetitive prismaticstructure is limited to the aforementioned preferable range. Hence,measures against the case where moire still occurs even if the pitch isin the aforementioned range comes into a question. According to thepresent invention, it is preferable to use a method in which theridgelines of the prismatic structures are formed to be inclined withrespect to the incidence side surface so that the prismatic structuresin the repetitive structure can be arranged to cross the pixels tothereby prevent moire, as illustrated in FIG. 3.

On this occasion, if the inclination angle θ2 to the incidence sidesurface is too large, deflection occurs in reflection by the opticalpath changing slopes A1. As a result, large deviation occurs in thedirection of changing of the optical path. This large deviation is aptto cause lowering of display quality. Therefore, the inclination angleθ2 of the ridgelines to the incidence side surface is selectedpreferably to be in a range of ±30 degrees, more preferably in a rangeof ±25 degrees, further preferably in a range of ±20 degrees.Incidentally, the symbol “±” means the direction of inclination of theridgelines with the incidence side surface as a reference. If theresolution of the liquid-crystal display panel is so low that moirenever occurs, or if moire is negligible, it is preferable that theridgelines are arranged to be as parallel with the incidence sidesurface as possible.

The optical film 1 maybe arranged so that the transparent film 11 andthe repetitive structure of the plurality of optical changing means Amay be formed integrally with the transparent film as illustrated inFIG. 1G. Alternatively, a separate layer which has the repetitivestructure of the plurality of optical changing means A and which is madeof a material the same as or different from that of the transparent film11 may be provided in close contact with the transparent film to formthe optical film 1, as illustrated in FIGS. 1A to 1F and FIG. 1H. Inaddition, in order to control the retardation or the like, thetransparent film 11 may be formed as at least two layers of overlaidbodies 11A and 11B which are made of the same kind of resin, ordifferent kinds of resins, as illustrated in FIG. 1H. That is, thetransparent film 11 does not have to be formed as an integrated singlelayer body by one kind of material, as illustrated in FIGS. 1A to 1F.Further, the transparent film may be made of a polarizer. On thatoccasion, a polarizer to be disposed in the liquid-crystal cellseparately can be omitted or reduced so that the liquid-crystal displaydevice can be made smaller in thickness. Although the thickness of thetransparent film can be determined suitably, it is generally set to benot thicker than 300 μm, particular in a range of from 5 to 200 μm, moreparticularly in a range of from 10 to 100 μm, from the point of view ofmaking the display device small in thickness.

The optical film having the optical path changing means can be formed bya suitable method. Examples of the suitable method include: a method inwhich a thermoplastic resin is pressed against a mold capable of forminga predetermined shape by heating to thereby transfer the shape; a methodin which a mold capable of forming a predetermined shape is filled witha hot-melted thermoplastic resin or a resin fluidized by heat or by asolvent; a method in which a fluid resin polymerizable by heat, byultraviolet rays or by radial rays such as electron rays is polymerizedin the condition that the fluid resin is filled or cast in a mold whichis capable of forming a predetermined shape; and so on. Theabove-mentioned methods are favorable particularly to a transparent filmwhich is formed integrally with optical path changing means so that thetransparent film and the repetitive structure of the optical pathchanging means are provided in the same body.

The preferable example of the method for forming the optical film havingthe plurality of optical path changing means is a method in which arepetitive prismatic structure having optical path changing slopes isgiven to one surface of a transparent film by a mold having thepredetermined prismatic structures. A specific example of the preferablemethod comprises the steps of: applying a curable resin polymerizable byultraviolet rays, radial rays, or the like, onto one surface of atransparent film; curing the curable resin by irradiation withultraviolet rays, radial rays, or the like, while bringing the coatinglayer into close contact with a surface of the mold on which thepredetermined prismatic structure is formed; and stripping off andcollecting the transparent film from the mold. Another specific exampleof the preferable method comprises the steps of: filling a mold with thecurable resin on the surface of the mold on which the predeterminedprismatic structure is formed; curing the resin layer by irradiationwith ultraviolet rays, radial rays, or the like while disposing atransparent film tightly on the resin layer; and stripping off andcollecting the transparent film from the mold. Thus, in the preferablemethod, a repetitive structure layer of a plurality of optical pathchanging means is formed on the transparent film as a separate layer.

In the latter specific example of the preferable method, the opticalpath changing means are added to the transparent film. In such a manner,if there is a large difference in refractive index between therepetitive structure layer of the added optical path changing means andthe transparent film, light exit efficiency maybe reduced largelybecause of interface reflection or the like. From the point of view toprevent the reduction of the light exit efficiency, it is preferablethat the difference in refractive index between the transparent film andthe repetitive structure layer of the optical path changing means ismade as small as possible, particularly larger than 0.10, moreparticularly not larger than 0.05. In addition, in that case, it ispreferable that the refractive index of the repetitive structure layerof the added optical path changing means is made higher than that of thetransparent film from the point of view of the light exit efficiency.Incidentally, a suitable transparent material corresponding to thewavelength range of incident light may be used to form the repetitivestructure layer of the optical path changing means, similarly to thecase with the transparent film.

Incidentally, the transparent film or the optical path changing meansmay be made of a suitable material exhibiting transparency andcorresponding to the wavelength range of light which is made to enterthe transparent film or the optical path changing means from theilluminator, or the like. Incidentally, examples of the suitablematerial used in a visible light range include: transparent resinrepresented by acrylic resin, polycarbonate resin, cellulose resin,norbornene resin, or the like; curable resin which can be polymerized byheat, by ultraviolet rays, or by radial rays such as electron rays; andso on.

Particularly, from the point of making the in-plane retardation notlarger than 30 nm, it is preferable to use a material exhibiting nobirefringence or low birefringence. In addition, internal stress may begenerated in the transparent film in the process of bonding. From thepoint of view of prevention of a retardation produced by such internalstress, it is preferable that a material small in photoelasticcoefficient is used. Although the thickness of the transparent film maybe determined suitably, it is generally made not thicker than 300 μm,particularly in a range of from 5 to 200 μm, more particularly in arange of from 10 to 100 μm from the point of view of making the opticalfilm thinner. Incidentally, the transparent film maybe formed as atleast two layers of overlaid bodies 11A and 11B which are formed out ofthe same kind of resin or different kinds of resins, as illustrated inFIG. 1H. That is, the transparent film does not have to be formed as anintegrated single layer body formed of one kind of material.

As illustrated in FIGS. 1A to 1H, the optical film is designed to haveadhesive means 12 (tacky layer) on the surface of the transparent film11 on which the repetitive prismatic structure 13 is not formed. Suchadhesive means 12 is to bond the optical film to a support member suchas a liquid-crystal display panel or the like. The bonding processthrough the adhesive means is performed for the following purposes:improvement of reflection efficiency by the optical path changing slopesA1 of the plurality of optical path changing means A; accordingly,improvement of luminance owing to effective utilization of the incidentlight on the side face; and so on. From the point of view of thesepurposes, according to the present invention, the adhesive means is setto have a refractive index of not lower than 1.49. If it is necessary torestrain total reflection on the bonded interfaces between the opticalfilm and the liquid-crystal display panel or the like to enhance theentrance efficiency of the light, which is transmitted through the panelto enter the optical film, and therefore to obtain a liquid-crystaldisplay device the display of which is bright and excellent inuniformity of brightness; the refractive index of the adhesive means ispreferably not lower than 1.50, more preferably not lower than 1.51,further preferably not lower than 1.52.

Incidentally, an optical glass plate is usually used as each cellsubstrate of the liquid-crystal cell. When the optical glass plate is anon-alkali glass plate, the refractive index of the non-alkali glassplate is generally in a range of from about 1.51 to about 1.52, and thebonding process is ideally performed through adhesive means which has ahigher refractive index than the above range. Thus, most of thetransmission light which has an angle large enough to make the lightenter the optical film from the cell can be made to enter the opticalfilm without being totally reflected on the bonded interfaces. If it isnecessary to restrain the loss of the quantity of light which occursbecause light cannot exit due to the trapping effect based on totalreflection, and if it is therefore necessary to improve displayluminance, uniformity of in-plane brightness, the difference inrefractive index in each of interfaces among light transmission typeoptical layers such as the adhesive means, the liquid-crystal cell, thetransparent film and so on is preferably not larger than 0.15, morepreferably not larger than 0.10, further preferably not larger than0.05. If the refractive index of the adhesive means or the transparentfilm were too high, the following problems are apt to arise: increase inloss of the quantity of light due to interface reflection caused by thelarge difference in refractive index, particularly increase inreflectivity of transmission light which is substantially in parallelwith the cell; increase in light absorption, particularly increase inabsorption of short-wavelehgth light of visible light; coloring becauseof wavelength dispersion, particularly increase of yellow degrees in thecase of ultraviolet-curing resin; deterioration in bonding properties ofthe adhesive layer or production of light absorption; and so on. Toavoid these problems, it is therefore preferable that the adhesive meansis set to have a refractive index of not higher than 1.6, particularlynot higher than 1.55, more particularly not higher than 1.53, and thetransparent film is set to have a refractive index of not higher than1.6, particularly not higher than 1.58, more particularly not higherthan 1.55, further particularly not higher than 1.53.

The adhesive means can be formed from a suitable adhesive agentexhibiting the aforementioned refractive index without any particularlimit. For example, an adhesive agent to be hardened by irradiation withultraviolet rays or radial rays or by heating can be used. From thepoint of view of handling properties such as facilitation of the bondingprocess, or from the point of view of stress relaxation ability tosuppress the internal stress from generating, a tacky layer ispreferably used as the adhesive means. A suitable tackiness agent can beused for the formation of the tacky layer. The suitable tackiness agentcontains, as a base polymer, a suitable polymer such as a rubberpolymer, an acrylic polymer, a vinyl-alkyl-ether polymer, a siliconepolymer, a polyester polymer, a polyurethane polymer, a polyetherpolymer, polyamide polymer, a styrene polymer, etc. Especially, atackiness agent excellent in transparency, weather resistance, heatresistance, etc. such as an acrylic tackiness agent containing, as abase polymer, a polymer mainly containing alkyl ester of acrylic acid ormethacrylic acid is used preferably to from the tacky layer.

As indicated by the arrow in FIG. 13, light may be enclosed by theoptical film due to interface reflection between the transparent filmand the tacky layer due to the difference in refractive indextherebetween. From the point of view of preventing the light from beingunable to exit from the optical film so as to suppress the loss of thequantity of light, a tacky layer having a refractive index different by0.12 or less, particularly different by 0.10 or less, more particularlydifferent by 0.05 or less from that of the transparent film ispreferably used. In this case, the tacky layer can be formed to be of alight diffusion type. Further, from the same point of view as mentionedabove, it is preferable that the difference in refractive index betweenthe tacky layer and the support member to which the tacky layer ispasted is not larger than 0.15, especially not larger than 0.10, moreespecially not larger than 0.05.

As transparent particles mixed with the adhesive means (tacky layer),there can be used one or two or more members suitably selected from thegroup consisting of inorganic particles of silica, alumina, titania,zirconia, tin oxide, indium oxide, cadmium oxide, antimony oxide, or thelike, which may be electrically conductive; and organic particles of acrosslinked or non-crosslinked polymer, or the like. In such a manner,the adhesive means may be made to be of a light diffusion type.Incidentally, a strip sheet 14 may be preferably temporarily bonded tothe adhesive means as shown in FIGS. 1A to 1H so that the adhesive meansis covered with the strip sheet 14 before the adhesive means is put intopractical use for the purpose of anti-contamination against depositionof foreign matters, etc.

A base material such as a sheet for the purpose of protecting theoptical path changing slopes may be disposed closely on a surface of thetransparent film on which the optical path changing means are formed. Asshown in FIGS. 7 to 9, the optical film may be also formed so that areflection layer 4 is disposed closely on a surface of the transparentfilm 11 on which the optical path changing means are formed. Thereflection layer is provided for reflecting and inverting light leakingfrom a surface of the transparent film on which the optical pathchanging slopes are formed, and provided for making the reflected andinverted light enter the optical film again. As a result, lightutilizing efficiency can be improved, so that a reflection-transmissiondouble type liquid-crystal display device can be formed.

The reflection layer can be formed of a suitable material such as awhite sheet similarly to the background art. Especially, a preferableexample of the high-reflectance reflection layer is constituted by: acoating layer containing powder of a high-reflectance metal such asaluminum, silver, gold, copper, chromium, etc., or alloy powder of sucha high-reflectance metal in a binder resin; a layer of theabove-mentioned metal or a dielectric multilayer film deposited by asuitable thin-film forming method such as a vacuum vapor depositionmethod, a sputtering method, or the like; a reflection sheet having thecoating layer or the deposited layer supported by a base material madeof a film, or the like; a sheet of metal foil; and so on. Thehigh-reflectance reflection layer is especially preferably used forforming a reflection-transmission double type liquid-crystal displaydevice.

The reflection layer to be formed may exhibit a light diffusingfunction. The reflection layer has a diffuse reflection surface fordiffusing the reflected light to thereby attain improvement in frontaldirectivity. When the reflection layer is formed by a surface rougheningprocess, the reflection layer can prevent the production of Newton ringsdue to its close contact to the film to thereby improve visibility. Theformation of the reflection layer of the light diffusion type can bemade by a method, for example, comprising the steps of: forming asurface of a film base material as a fine prismatic structure by asuitable method such as a surface roughening method using sandblasting,matting, or the like, or a particle adding method; and providing areflection layer on the film base material so that the fine prismaticstructure is reflected in the reflection layer. The reflection layerhaving such a fine prismatic structure to reflect the fine prismaticstructure on the surface of the film base material can be formed byproviding a metal on the surface of the film base material by a suitablevapor deposition or plating method such as a vacuum vapor depositionmethod, an ion-plating method, a sputtering method, or the like.

The optical film according to the present invention has optical pathchanging slopes by which the optical path of the light incident on theside surface from the illuminator, or the like, or the transmissionlight of the incident light is changed to a direction excellent inperpendicularity in favor of viewing. Hence, light can be made to exitwith good light utilizing efficiency. Moreover, the optical filmexhibits good transmittance to external light. When the optical film isdisposed on the visual side (front side) or opposite side (back side) ofthe liquid-crystal display P provided with the illuminator 5 or 51disposed on at least one side of the liquid-crystal display panel P asillustrated in FIGS. 8 and 9, it is possible to form various devicessuch as a transmission type liquid-crystal display device which isbright and easy to view, a reflection-transmission double typeliquid-crystal display device which is excellent in power saving, or thelike.

Incidentally, in the liquid-crystal display device, a great part of thelight incident on the incident side surface from the illuminator 5 or 51is reflected through the upper and lower cell substrates 21 and 28 inaccordance with the law of refraction on the basis of thicknessproportion of respective layers in the liquid-crystal display panel asindicated by the arrows in FIG. 7. That is, most of the incident lightis reflected at a total reflection angle of about ±42 degrees on a glasssubstrate having a refractive index of 1.5. Thus, the light istransmitted backward efficient so as to prevent from exiting (leaking)from the surface of the panel. At this time, the optical path of thelight incident on the optical path changing slopes A1 of the opticalfilm 1 is efficiently changed to the viewing direction, that is, to thefrontal direction. Hence, display excellent in uniformity of brightnesson the whole surface of the panel display screen can be achieved. Then,the optical film is formed to have layers of adhesive means, atransparent film and optical path changing means which have a refractiveindex equal to or larger than that of the cell substrate on the sidewhere the optical film is disposed. As a result, in-cell transmissionlight enters the optical film efficiently without being totallyreflected on the interfaces among the layers and the cell substrate.

In the above description, a suitable transmission type liquid-crystaldisplay panel including a liquid-crystal cell can be used as theliquid-crystal display panel P. That is, as illustrated in FIGS. 8 and9, it is possible to use, as the liquid-crystal display panel P, atransmission type liquid-crystal display panel which has liquid crystal25 enclosed by cell substrates 21 and 28 through a sealing material 24so that the incident light from a side of arrangement of the opticalfilm 1 is made to exit as display light from the other side throughcontrol of the liquid crystal, or the like. The liquid-crystal displaypanel P is not particularly limited in kind.

Incidentally, specific examples of the liquid-crystal cell include atwisted or non-twisted cell such as a TN liquid-crystal cell, an STNliquid-crystal cell, an IPS liquid-crystal cell, an HAN cell, an OCBcell, or a VA liquid-crystal cell; a guest-host or ferroelectricliquid-crystal cell; a light diffusion type liquid-crystal cell such asan internal diffusion type liquid-crystal cell; and soon. Further, asuitable drive method such as an active matrix method or a passivematrix method may be used as the method for driving liquid crystal. Asillustrated in FIGS. 8 and 9, the liquid crystal is generally driventhrough transparent electrodes 22 and 27 provided on the inner surfacesof the pair of cell substrates 21 and 28.

A suitable transparent substrate such as a glass substrate or a resinsubstrate can be used as each of the cell substrates. Especially, atransparent substrate made of an optically isotropic material ispreferably used from the point of view of display quality, etc. Asubstrate such as a non-alkali glass plate exhibiting excellentcolorlessness and transparency with respect to a blue glass plate ispreferably used from the point of view of improvement of luminance anddisplay quality, etc. A resin substrate is preferably used from thepoint of view of reduction in weight, etc. A blue glass plate having arefractive index in a range of about 1.47 to 1.49 is often used as acell substrate in a passive driving TN or STN cell. In that case, if anoptical film constituted by adhesive means and a transparent film eachhaving a refractive index of not lower than 1.49 is used, totalreflection on the interfaces can be prevented so that reflection loss ofthe transmission light can be reduced. On the other hand, theabove-mentioned non-alkali glass plate is often used as a cell substratein an active matrix type TFT or TFD cell or the like after asemiconductor film is provided in the cell necessarily. In that case,when an optical film which is constituted by adhesive means and atransparent film each having a refractive index of not lower than 1.50is used, the angle with which total reflection takes place can bereduced so that reflection loss of transmission light can be reduced.Also in the case of the non-alkali glass plate used in the TFT and TFDcell, when an optical film which is constituted by adhesive means and atransparent film each having a refractive index of not lower than 1.51is used, total reflection on the interfaces can be prevented so thatreflection loss of transmission light can be reduced. Incidentally, inthe case of a resin substrate, for example, in the case of an epoxyresin substrate having a refractive index of about 1.51, an optical filmsimilar to that in any one of the aforementioned cases of the non-alkaliglass plate can be used preferably. The thickness of the cell substratecan be determined suitably in accordance with enclosing strength ofliquid crystal, or the like, without any particular limitation. Thethickness of the cell substrate is generally selected to be in a rangeof from 10 μm to 5 mm, especially in a range of from 50 μm to 2 mm, moreespecially in a range of from 100 μm to 1 mm, from the point of view ofbalance between light transmission efficiency and reduction in thicknessand weight.

When the liquid-crystal cell is formed, one suitable functional layer,or, two or more suitable functional layers maybe provided as occasiondemands. Examples of such a suitable functional layer include an alignedfilm made of a rubbed film for aligning the liquid crystal, a colorfilter for color display, and so on. Incidentally, aligned films 23 and26 are generally formed on transparent electrodes 22 and 27 respectivelyas shown in FIGS. 8 and 9. A color filter not shown is generallyprovided between one of the cell substrates 21 and 28 and correspondingone of the transparent electrodes.

One suitable optical layer, or, two or more suitable optical layers suchas polarizer 31 and 34, retarders 32 and 33, a light diffusing layer maybe added to the liquid-crystal display panel, as illustrated in FIGS. 8and 9. The polarizers are provided for achievement of display by usinglinearly polarized light. The retarders are provided for improvement ofdisplay quality by compensation for retardation due to birefringence ofliquid crystal, etc. The light diffusing layer is provided for thefollowing purposes: enlargement of a display range by diffusion ofdisplay light, uniformity of luminance by leveling of emission-line-likelight emission through slopes of the optical film, increase of thequantity of light entering the optical film by diffusion of transmissionlight in the liquid-crystal display panel, etc.

A suitable plate can be used as the polarizer without any particularlimitation. From the point of view of obtaining good-contrast-ratiodisplay due to incidence of highly linearly polarized light, etc., afilm high in the degree of polarization may be preferably used. Examplesof the preferable film include: an absorption type polarizing film madeof a drawn film having a dichromatic material such as iodine ordichromatic dye adsorbed on a hydrophilic macromolecular film such as apolyvinyl alcohol film, a partially formalized polyvinyl alcohol film ora partially saponified ethylene-vinyl acetate copolymer film; a film inwhich a transparent protective layer is provided on one or either sideof the aforementioned absorption type polarizing film, or the like.

A material excellent in transparency, mechanical strength, thermalstability, moisture shielding characteristic, etc. is preferably usedfor the formation of the transparent protective layer. Examples of thematerial include: polymers such as acetate resin, polyester resin,polyether-sulfone resin, polycarbonate resin, polyamide resin, polyimideresin, polyolefin resin, acrylic resin, polyether resin, polyvinylchloride resin, styrene resin and norbornene resin; heat-curable orultraviolet-curable resins such as acrylic resin, urethane resin,acrylic urethane resin, epoxy resin, silicone resin, etc.; and so on.The transparent protective layer may be bonded as a film by a bondingmethod or may be applied as polymer liquid by a coating method, and soon.

The polarizer to be used, especially the polarizer disposed on thevisual side liquid-crystal display panel may be subjected to non-glaretreatment or anti-reflection treatment for preventing viewing from beingdisturbed by surface reflection of external light. Non-glare treatmentcan be made to form a surface of the polarizer as a fine prismaticstructure. In the non-glare treatment, various methods may be used forforming a surface of the polarizer as a fine prismatic structure.Examples of the methods include: a surface roughening method such as asandblasting method, an embossing method, etc.; a method of mixingtransparent particles such as silica particles; and so on.Anti-reflection treatment can be made by a method of forming a coherentvapor deposition film, or the like. Alternatively, non-glare treatmentor anti-reflection treatment can be made by a method of bonding a filmhaving a surface structure of fine prismatic structures or aninterference film. Incidentally, two polarizers may be provided onopposite sides of the liquid-crystal cell respectively, as shown in theFIGS. 8 and 9, or one polarizer may be provided on only one side of theliquid-crystal cell.

On the other hand, each of the retarders may be formed of a suitablematerial. Examples of the material include a birefringence film obtainedby drawing a film of a suitable polymer as illustrated in thedescription of the transparent protective layer by a suitable methodsuch as monoaxial drawing or biaxial drawing; an aligned film of asuitable liquid-crystal polymer such as a nematic liquid-crystal polymeror a discotic liquid-crystal polymer, and an aligned layer of thealigned film supported by a transparent base material. A material havinga refractive index controlled in a direction of thickness under theoperation of heat shrinkage force of a heat-shrinkable film may be alsoused.

The compensatory retarders 32 and 33 shown in FIGS. 8 and 9 aregenerally disposed between the back side polarizer 31 and theliquid-crystal cell and between the visual side polarizer 34 and theliquid-crystal cell, respectively, as occasion demands. A suitablematerial can be used as each of the retarders corresponding to thewavelength range, etc. Each of the retarders may be formed of a laminateof two or more layers in order to control optical characteristic such asretardation, etc.

A coating layer, a diffusing sheet, or the like, having a surfacestructure of fine prismatic structures similarly to that of thenon-glare layer can be used to form the light diffusing layer by asuitable method. The light diffusing layer may be disposed as anadhesive layer 35 prepared in the same manner as the transparentparticles-containing adhesive layer 12. In this case, the lightdiffusing layer can serve also as an adhesive layer 35 for bonding thepolarizer 34 and the retarder 32 to each other, as shown in FIGS. 8 and9. Hence, reduction in thickness can be achieved. Although the lightdiffusing layer can be disposed on the outer side (visual side) of thepolarizer, arrangement of the light diffusing layer on theliquid-crystal cell side as shown in FIGS. 8 and 9 is more favorablethan arrangement of the light diffusing layer on the polarizer 34 side.This is because external light is made incident on the light diffusinglayer after absorption by the polarizer so that reflection loss causedby backward scattering through the light diffusing layer can besuppressed.

On the other hand, the illuminator disposed on one of side surfaces ofthe liquid-crystal display panel is provided so that light to beutilized as light for illuminating the liquid-crystal display device ismade to incident on the side surface of the liquid-crystal displaypanel. Hence, reduction in thickness and weight of the liquid-crystaldisplay device can be achieved when the illuminator is used incombination with the optical film disposed on the back or front side ofthe panel. A suitable illuminator can be used as the illuminator.Examples of the illuminator preferably used include a linear lightsource such as a (cold or hot) cathode tube, a point light source suchas a light-emitting diode, an array of point light sources arranged inline or plane, and a combination of a point light source and a linearlight pipe for converting the incident light from the point light sourceinto light of a linear light source through the linear light pipe.

One illuminator 5 may be disposed on one of side surfaces of theliquid-crystal display panel P as shown in FIG. 8, or illuminators 5 and51 maybe disposed on two or more side surfaces of the liquid-crystaldisplay panel P as shown in FIG. 9. When illuminators are disposed on aplurality of side surfaces, the plurality of side surfaces may beprovided as a combination of side surfaces opposite to each other asshown in FIG. 9, or may be provided as a combination of side surfacescrossing each other. Further, the plurality of side surfaces may beprovided as a combination of three or more side surfaces by use of theaforementioned combinations together.

The illuminator makes it possible to view the liquid-crystal displaydevice in a transmission mode in which the illuminator is switched on.When the liquid-crystal display device is provided as areflection-transmission double type liquid-crystal display device, theilluminator can be switched on/off because the illuminator is notnecessary to be switched on when the display device is viewed in areflection mode by using external light. Any suitable method can be usedfor switching on/off the illuminator. Any one of background-art methodsmay be used. Incidentally, the illuminator may be of a multicolor lightemission type in which the color of emitted light can be changed. Ordifferent types of illuminators maybe provided so that multicolor lightemission can be made through the different types of illuminators.

As shown in FIGS. 8 and 9, each of the illuminators 5 and 51 may be usedin combination with a suitable assisting means such as a reflector 52.The reflector 52 is provided for enclosing the illuminator to leadscattered light to side surfaces of the liquid-crystal display panel Pas occasion demands. A suitable reflection sheet such as a resin sheetprovided with a high-reflectance metal thin film, a white sheet, a sheetof metal foil, etc. may be used as the reflector. The reflector may beused also as a fixing means for enclosing the illuminator by a method ofbonding end portions of the reflector to end portions of the cellsubstrates of the liquid-crystal display panel correspondingly.

In the present invention, optical devices or parts such as aliquid-crystal cell, a polarizer, a retarder, etc. for forming theliquid-crystal display device may be wholly or partially integrallylaminated/fixed onto one another or may be may be disposed separably.From the point of view of prevention of lowering of contrast bysuppression of interface reflection, etc., it is preferable that suchoptical devices or parts are fixed onto one another. A suitable adhesivemeans such as a tackiness agent can be used for the closely fixing thesedevices or parts. The suitable adhesive layer may contain transparentparticles, etc., as described above so as to be an adhesive layerexhibiting a diffusing function.

The optical devices or parts, especially visual side optical devices orparts, maybe formed to have ultraviolet-ray absorbing power by a methodof treatment with an ultraviolet-ray absorbent such as a salicylic estercompound, a benzophenone compound, a benzotriazole compound, acyanoacrylate compound, a nickel complex salt compound, etc.

EXAMPLE 1

A mold processed into a predetermined shape in advance was filled withacrylic ultraviolet-curable resin (ARONIX UV-3701, made by TOAGOSEI Co.,Ltd.) by dropping with a dropper. An 80 μm-thick triacetylcellulose(TAC) film (having a saponified surface) was disposed quietly on theresin, and made to adhere closely to the resin by a rubber roller so asto eliminate excessive resin and bubbles. The TAC film with the resinwas irradiated with ultraviolet rays by a metal halide lamp so as to becured. The resin-including TAC film cured thus was stripped off from themold and cut into a predetermined size. Thus, a transparent film wasobtained so that a layer of a plurality of optical path changing meanshaving a refractive index of 1.533 was formed on the TAC film having arefractive index of 1.49. Then, a tacky layer with a refractive index of1.47 was provided on the other surface of the transparent film on whichthe plurality of optical path changing means were not formed. Thus, anoptical film was obtained.

Incidentally, the optical film was 60 mm wide and 45 mm deep, and hadprism-like concave portions formed at intervals of a pitch of 210 μm(FIG. 1C). The ridgelines of the concave portions were continued andparallel with one another in the widthwise direction of the opticalfilm. The inclination angles of optical path changing slopes A1 of theconcave portions vary in a range of from 42.5 to 43 degrees while theinclination angles of gentle slopes A3 thereof vary in a range of from1.8 to 3.5 degrees, so that the difference in inclination angle betweenadjacent ones of gentle slopes was not larger than 0.1 degrees. Theprojected width of each of the optical path changing slopes on the filmplane was in a range of from 10 to 16 μm, and the ratio of the projectedarea of the gentle slopes on the film plane to the projected area of theoptical path changing slopes on the film plane was not smaller than 12times.

Next, a tacky layer containing resin particulates was provided on a TACfilm so as to form a light diffusing film. The light diffusing film wasbonded onto the visual side of a TN type liquid-crystal cell which wasalready available on the market, while polarizers were pasted on thefront and back sides of the cell. Thus, a normally white transmissiontype TN liquid-crystal display panel was formed. A cathode-ray tube wasdisposed on one of side surfaces of the liquid-crystal display panel,and enclosed by a reflector of a silver-vapor-deposited reflectionsheet. Opposite end portions of the reflector were bonded to the upperand lower surfaces of the panel so that the cathode-ray tube was fixed.Then, the aforementioned optical film was bonded onto the polarizer onthe back side (opposite to the visual side) of the panel through thetacky layer of the optical film so that the optical path changing slopeswere parallel to and faced the cathode-ray tube. A reflection sheet madeof a white polyester film is disposed on the back side of the opticalfilm. Thus, a reflection-transmission double type liquid-crystal displaydevice was obtained.

EXAMPLE 2

A transmission type liquid-crystal display device was obtained by usingan optical film in the manner similar to that in Example 1, except thatthe optical film was formed to have a plurality of optical path changingmeans (FIG. 1B) each of which had an optical path changing slope A1inclined at an angle of about 42 degrees, a steep slope A2 having avertex angle of 70 degrees with the optical path changing slope A1, anda flat surface A4 having a projected area not smaller than 10 times aslarge as the total projected area of the optical path changing slope A1and the steep slope A2 on the film plane.

EXAMPLE 3

A transmission type liquid-crystal display device was obtained by usingan optical film (FIG. 6) in the manner similar to that in Example 1,except that the optical film was formed to have a plurality of opticalpath changing means each having a length of 80 μm (FIG. 1B). Each of theplurality of optical path changing means had an optical path changingslope A1 which had an inclination angle of about 42 degrees and whichhad a projected width of 10 μm on the film plane, and a steep slope A2which had an inclination angle of about 55 degrees. The plurality ofoptical path changing means were disposed so that the lengths of theplurality of optical path changing means were substantially parallelwith one another in the widthwise direction of the optical film, and sothat the density of the optical path changing means became graduallyhigher as the location went farther from the side surface, on which thelight is incident, in the widthwise direction of the optical film.Incidentally, the area of each of the flat surfaces A4 was not smallerthan 10 times as large as the total projected area of the correspondingoptical path changing slope A1 and the corresponding steep slope A2 onthe film plane.

EXAMPLE 4

A transmission type liquid-crystal display device was obtained by usingan optical film (FIG. 4) in the manner similar to that in Example 1,except that the optical film was formed to have a plurality of opticalpath changing means each having a length of 80 μm (see FIG. 1A) andexcept that cathode-ray tubes were disposed on opposite side surfaces ofthe optical film. Each of plurality of optical path changing means wasshaped like an isosceles triangle by two optical path changing slopes A1each of which had an inclination angle of about 42 degrees and aprojected width of 10 μm on the film plane. The plurality of opticalpath changing means were disposed at random so that the lengths of theoptical path changing means were parallel with one another in thewidthwise direction of the optical film, and so that the density of theplurality of optical path changing means became gradually higher as thelocation went toward the center portion from the side surface, on whichthe light is incident, in the widthwise direction of the optical film.Incidentally, the projected area of each of the flat surfaces A4 was notsmaller than 10 times as large as the total projected area of every twoof the optical path changing slopes A1 on the film plane.

EXAMPLE 5

A transmission type liquid-crystal display device in atwo-incidence-side-surface system was obtained by using an optical filmin the manner similar to that in Example 4, except that the optical filmwas formed to have a plurality of optical path changing means (see FIG.1E). Each of plurality of optical path changing means had a length of80μm, was constituted by a groove shaped substantially like an tetragonin section. In the groove, there were two optical path changing slopesA1 each of which had an inclination angle of about 42 degrees and aprojected width of 10 μm on the film plane. The plurality of opticalpath changing means were disposed at random so that the lengths of theoptical path changing means were substantially parallel with one anotherin the widthwise direction of the optical film, and so that the densityof the plurality of optical path changing means became gradually higheras the location went toward the center portion from the side surface, onwhich the light is incident, in the widthwise direction of the opticalfilm. Incidentally, the projected area of each of the flat surfaces A4was not smaller than 10 times as large as the total projected area ofeach of the optical path changing means on the film plane.

EXAMPLE 6

A reflection-transmission double type liquid-crystal display device wasobtained by using an optical film in the manner similar to that inExample 2, except that the optical film had a reflection layer of asilver-vapor-deposited film disposed on the surface of the optical filmon which the optical path changing means were formed, and except thatreflector on the back side was omitted.

Comparative Example 1

A transmission type liquid-crystal display device (FIG. 14) was obtainedin the manner similar to that in Example 1, except that the optical filmwas replaced by a scattering sheet having a surface roughened bysandblasting. Incidentally, the scattering sheet was disposed so thatthe roughened surface was on the back side (opposite to the visualside).

Comparative Example 2

A transmission type liquid-crystal display device was obtained by usingan optical film in the manner similar to that in Example 1, except thatthe optical film was formed to have a plurality of optical path changingmeans (FIG. 1B) each of which had an optical path changing slope A1inclined at an angle of about 30 degrees, a steep slope A2 having avertex angle of 70 degrees with the optical path changing slope A1, anda flat surface A4 having a projected area not smaller than 10 times aslarge as the total projected area of the optical path changing slope A1and the steep slope A2 on the film plane.

Comparative Example 3

A transmission type liquid-crystal display device was obtained in thefollowing manner. A cathode-ray tube was disposed on one of sidesurfaces of a light pipe which had a embossed rough surface on the backside (opposite to the visual side) and which was 1.2 mm thick. Thecathode-ray tube was enclosed by a reflector made of asilver-vapor-deposited reflection sheet. Opposite end portions of thereflector were bonded onto the upper and lower surfaces of the lightpipe. The light pipe obtained thus was disposed on a reflector made of awhite polyester film, and a normally white transmission type TNliquid-crystal panel which was already available on the market wasdisposed on the light pipe through a light scattering plate.

Comparative Example 4

A reflection-transmission double type liquid-crystal display device wasobtained in the manner similar to that in Example 6, except that ascattering film in Comparative Example 1 which had a reflection layer ofa silver-vapor-deposited film disposed on the scattering surface wasused and except that the reflector on the back side was omitted.

Comparative Example 5

A reflection-transmission double type liquid-crystal display device wasobtained in the manner similar to that in Example 6, except that anoptical film in Comparative Example 2 had a reflection layer of asilver-vapor-deposited film disposed on the surface on which opticalpath changing means were formed, and except that the reflector on theback surface side was omitted.

Evaluation Test 1

Frontal luminance in the center portion of the transmission type orreflection-transmission type liquid-crystal display device obtained ineach of Examples 1 to 6 and Comparative Examples 1 to 5 was measured bya luminance meter (BM-7 made by TOPCON Corp.) in a transmission mode inwhich the cathode-ray tube(s) was (or were) switched on in the conditionthat no voltage was applied to the liquid-crystal display panel. Inaddition, frontal luminance was also measured in a white state in areflection mode in which the cathode-ray tube(s) was (or were) switchedoff while external light by using ring-like illumination was madeincident at an angle of 15 degrees. Results of the measurement wereshown in the following Table 1.

Frontal luminance (cd/m²) transmission mode reflection mode Example 1 20— Example 2 20 — Example 3 24 — Example 4 36 — Example 5 36 — Example 620 436 Comp. Example 1  4 — Comp. Example 2  8 — Comp. Example 3 34 —Comp. Example 4  4 386 Comp. Example 5 10 410

It is apparent from the Table 1 that frontal luminance in each ofExamples 1 to 6 superior to that in any one of Comparative Examples 1,2, 4 and 5 was attained. This is because light was made to exit in adirection reverse to the light source in the transmission mode inComparative Examples 1, 2, 4 and 5 so that the exit light wasinsufficient in frontal luminance so as to be difficult to contribute todisplay. Particularly in Comparatives 1 and 4, the exit light wasinsufficient in any direction.

On the other hand, in Examples 4 and 5, it is apparent that luminancewas enhanced conspicuously due to the two-light-source system so thatbrightness surpassed that in Comparative Example 3 in which aside-lighting type light pipe was used. Incidentally, in the system ofusing the side-lighting light pipe in Comparative Example 3, increase inthickness of the liquid-crystal display device due to the light pipeappeared conspicuously so that it was difficult to reduce the thicknessof the liquid-crystal display device. Further, each of Examples 1 to 6had no problem in viewing in the transmission mode in which a voltagewas applied to the liquid-crystal display panel, so that excellentdisplay quality was ensured. In addition, in Example 2, easy viewing inthe case of removing the light diffusing film was inferior to that inthe case of providing the light diffusing film. However, frontalluminance in the above-mentioned cases stood comparison with each other.

On the other hand, although display in the reflection mode in each ofExample 6 and Comparative Examples 4 and 5 had no image disorder or thelike in the condition that the liquid-crystal display panel was suppliedwith a voltage, display in Comparatives 4 and 5 was darker than that inExample 6. It is understood from the above description that brightdisplay was attained in the transmission mode in each of Embodiment 1 to6, and bright display was attained also in the reflection mode inEmbodiment 6. It is therefore proved that, according to the presentinvention, it is possible to form a transmission type orreflection-transmission double type liquid-crystal display device whichis prevented from increasing in bulk and weight caused by a light pipe,and which is hence made small in thickness and light in weight by a filmsystem, and which is good in display quality.

EXAMPLE 7

A mold processed into a predetermined shape in advance was filled withacrylic ultraviolet-curing resin (ARONIX UV-3701, made by TOAGOSEI Co.,Ltd.) by dropping with a dropper. An 80 μm-thick triacetylcellulose(TAC) film (having a saponified surface) was disposed quietly on theresin, and made to adhere closely to the resin by a rubber roller so asto eliminate excessive resin and bubbles. The TAC film with the resinwas irradiated with ultraviolet rays by a metal halide lamp so as to becured. The resin-including TAC film cured thus was stripped off from themold and cut into a predetermined size. Thus, a transparent film wasobtained so that a layer of a plurality of optical path changing meanshaving a refractive index of 1.533. was formed on one surface of the TACfilm having a refractive index of 1.49. Then, a tacky layer with arefractive index of 1.47 was provided on the other surface of thetransparent film on which the plurality of optical path changing meanswere not formed. Thus, an optical film was obtained. A reflection layerformed of a silver-vapor-deposited film was provided on the surface ofthe optical film on which the optical path changing means were formed.

The average in-plane and thicknesswise retardations of the optical filmwere 6 nm and 44 nm, respectively. The optical film was 60 mm wide and45 mm deep, and had optical path changing means (FIG. 1B) formingcontinuous grooves at intervals of a pitch of 210 μm. The ridgelines ofthe grooves were parallel with one another in the width direction of theoptical film. In each of the optical path changing means, an opticalpath changing slope A1 was about 42 degrees in inclination angle, 10 to16 μm in width, and 70 degrees in vertex angle with a steep slope A2. Ineach of the plurality of optical path changing means, the projected areaof a flat surface portion A4 was not smaller than 10 times as large asthe total projected area of the corresponding optical path changingslope A1 and the corresponding steep slope A2 on the film plane.

Next, a tacky layer containing resin particulates was provided on a TACfilm so as to form a light diffusing film. The light diffusing film wasbonded onto the visual side of a TN type liquid-crystal cell which wasalready available on the market, while polarizers were pasted on thefront and back sides of the cell. Thus, a normally white transmissiontype TN liquid-crystal display panel was formed. A cathode-ray tube wasdisposed on one of side surfaces of the liquid-crystal display panel,and enclosed by a reflector of a silver-vapor-deposited reflectionsheet. Opposite end portions of the reflector were bonded to the upperand lower surfaces of the panel so that the cathode-ray tube was fixed.Then, the aforementioned optical film was bonded onto the polarizer onthe back side (opposite to the visual side) of the panel through thetacky layer of the optical film so that the optical path changing slopeswere parallel to and faced the cathode-ray tube. Thus, areflection-transmission double type liquid-crystal display device wasobtained.

EXAMPLE 8

An optical film the average in-plane and thicknesswise retardations ofwhich were 2 nm and 32 nm respectively was obtained in the mannersimilar to that in Example 7, except that the thickness of the TAC filmwas made 40 μm. A reflection-transmission double type liquid-crystaldisplay device was obtained by using the thus obtained optical film.

Comparative Example 6

A reflection-transmission double type liquid-crystal display device(FIG. 14) was obtained in the manner similar to that in Example 7,except that the optical film was replaced by a scattering film having asurface roughened by sandblasting. The average in-plane andthicknesswise retardations in the roughened surface of the scatteringfilm were 5 nm and 47 nm respectively, and a tacky layer was provided onthe smooth surface of the scattering film.

Comparative Example 8

A reflection-transmission double type liquid-crystal display device wasobtained by using an optical film in the manner similar to that inExample 7, except that the optical film was formed so that theinclination angle of each of optical path changing slopes was about 30degrees, and the average in-plane and thicknesswise retardations of theoptical film were 5 nm and 44 nm respectively.

Comparative Example 9

A reflection-transmission double type liquid-crystal display device wasobtained by using an optical film in the manner similar to that inExample 7, except that the TAC film was replaced by a biaxially drawnpolyester film, and the average in-plane and thicknesswise retardationsof the optical film were 2,150 nm and 5,820 nm respectively.

Comparative Example 9

A reflection-transmission double type liquid-crystal display device wasobtained in the following manner. A cathode-ray tube was disposed on oneof side surfaces of a light pipe which had an embossed rough surface onthe back side (opposite to the visual side) and which was 1.2 mm thick.The cathode-ray tube was enclosed by a reflector constituted by asilver-vapor-deposited reflection sheet. The opposite end portions ofthe reflector were bonded with the upper and lower surfaces of the lightpipe. The light pipe obtained thus was disposed on a reflection sheetmade of a silver-vapor-deposited polyester film, and a TN liquid-crystalpanel in the manner similar to that in Example 7 was disposed on thelight pipe.

Evaluation Test 2

Frontal luminance in the center portion of the reflection-transmissiondouble type liquid-crystal display device obtained in each of Examples 7and 8 and Comparative Examples 6 to 9 was measured by a luminance meter(BM-7 made by TOPCON Corp.) in a transmission mode where the cathode-raytube was switched on in the condition that no voltage was applied to theliquid-crystal display panel. In addition, frontal luminance was alsomeasured in a white state in a reflection mode where the cathode-raytube was switched off while external light by using ring-likeillumination was made incident at an angle of 15 degrees.

Results of the measurement were shown in the following Table 2.

Frontal luminance (cd/m²) Ex. 7 Ex. 8 Comp. 6 Comp. 7 Comp. 8 Comp. 9Trans- 22 24 6 8 18 35 mission mode Reflection 416 432 365 388 124 360mode

It is apparent from the Table 2 that frontal luminance in each ofExamples 7 and 8 superior to that in any one of Comparative Examples 6to 8 was attained in both the transmission mode and the reflection mode.Light was made to exit in a direction reverse to the light source in thetransmission mode in Comparative Examples 6 and 7, so that the exitlight for frontal luminance was so insufficient as to be difficult tocontribute to display. Particularly in Comparative Example 6, the exitlight was insufficient in any direction. On the other hand, inComparative Example 8, a stripe-like color shift appeared in aperspective direction so that uniform display could not be obtained.

Further, in the transmission mode, Examples 7 and 8 had excellentdisplay quality without any problem in viewing in the condition that avoltage was applied to the liquid-crystal display panel. On thecontrary, in Comparative Examples 6 and 7, display was too dark to vieweasily. In Comparative Example 8, a color shift appeared in perspectiveview due to slight coloring so that display was not easy to view. Inaddition, in Example 7, easy viewing in the case where the lightdiffusing film was removed was inferior to that in the case where thelight diffusing film was provided. However, frontal luminance in bothcases stood comparison with each other.

On the other hand, also in the reflection mode, display in Examples 7and 8 and Comparative Examples 6 and 7 had bright and excellent displaywith no image disorder or the like in the condition that a voltage wasapplied to the liquid-crystal display panel. On the contrary, inComparative. Example 8, coloring was present in display and astripe-like color shift appeared in perspective view. In ComparativeExample 9, a parallax appeared in an image due to the thickness of thelight pipe so that the image was not easy to view. Incidentally, in themethod of using the side-lighting type light pipe in Comparative Example9, increase in thickness of the display device appeared conspicuouslydue to the light pipe so that reduction in thickness of the displaydevice was difficult to be attained.

It is proved from the above description, where bright display wasattained in both the reflection mode and the transmission mode inExamples 7 and 8. Accordingly, according to the present invention, it ispossible to form a transmission-type or reflection-transmission doubletype liquid-crystal display device which is prevented from increasing involume and weight caused by a light pipe, and which is made small inthickness and light in weight by a film system, and which is good indisplay quality.

EXAMPLE 9

A mold processed into a predetermined shape in advance was filled withacrylic ultraviolet-curing resin (GRANDIC RC-8720, made by DAINIPPON INK& CHEMICALS Inc.) by dropping with a dropper. A non-drawn polycarbonate(PC) film 60 μm thick was disposed quietly on the resin, and made toadhere closely to the resin by a rubber roller as so to eliminateexcessive resin and bubbles. The PC film with the resin was irradiatedwith ultraviolet rays at 300 mJ/cm² by a metal halide lamp so as to becured. The resin-including PC film cured thus was stripped off from themold and cut into a predetermined size. A repetitive structure layer ofa plurality of optical path changing means having a refractive index of1.522 was formed on one surface of the PC film. Then, the PC film wasstripped off to obtain a transparent film. A rubber tacky layer with arefractive index of 1.515 provided on a strip sheet was provided on theother surface of the transparent film which did not have the pluralityof optical path changing means.

The optical film was 50 mm width and 50 mm deep, and had continuousgrooves at intervals of a pitch of 210 μm. The ridgelines of the grooveswere parallel with the width direction of the optical film. Aninclination angle and a width of each of optical path changing slopes A1of the grooves were in a range of from 42.5 to 43 degrees and in a rangeof from 10 to 16 μmm, respectively. An inclination angle of each ofgentle slopes A3 (flat surfaces) was in a range of from 1.8 to 3.5degrees, and the difference in inclination angle between adjacent onesof gentle slopes A3 was not larger than 0.1 degrees, and the projectedarea of the gentle slopes on a plane of the optical film was not smallerthan 12 times as large as that of the optical path changing slopes A1 onthe film plane (FIG. 1C). Next, the optical film was stripped off fromthe strip sheet, and bonded onto the back side (opposite to the visualside) of a liquid-crystal display panel through the tacky layer of theoptical film so as to obtain a liquid-crystal display device.

The liquid-crystal display panel was a polymer dispersion type obtainedas follows. A non-alkali glass plate having a refractive index of 1.51by abrasive finishing was subjected to plasma treatment in an argonatmosphere. Transparent electrodes of indium-tin oxide (ITO) thin filmswere formed on the non-alkali glass plate by sputtering. A pair of cellsubstrates obtained thus by the transparent electrodes were disposedwith a gap through gap adjusters made of spherical glass beads and fixedwith sealers, so that the transparent electrodes were opposite to eachother. Then, a uniform mixture liquid of 10 parts (parts by weight, thisrule was also applied hereinafter) of trimethyl propane triacrylate, 10parts of 2-hydroxyethyl acrylate, 25 parts of acrylic oligomer (M-1200,made by TOAGOSEI Co., Ltd.), 0.5 parts of photo-curing starter (Darocure1173, made by MERCK & Co., Inc.), and 50 parts of liquid crystal (E7,made by BDH INDUSTRIES Ltd.) was injected into the gap between the cellsubstrates, and irradiated with ultraviolet rays from the outside of thecell so as to form a liquid-crystal cell. An anti-reflection film wasfurther bonded onto the visual side of the liquid-crystal cell through arubber tacky layer produced in the above-mentioned manner so that theanti-reflection layer was positioned outside. Thus, a liquid-crystaldisplay panel of the polymer dispersion type was obtained. Incidentally,the transparent electrode on each cell substrate was divided into two inadvance.

Next, a cathode-ray tube was disposed on one of side surfaces of theliquid-crystal display panel, and enclosed by a reflector made of asilver-vapor-deposited reflective sheet. The opposite end portions ofthe reflector were bonded to the upper and lower surfaces of the panelso that the cathode-ray tube was fixed. Thus, a transmission typeliquid-crystal display device in which the illuminator was disposed wasformed. The liquid-crystal display device was disposed on a blackboard.Incidentally, the optical film was disposed so that the optical pathchanging slopes face and are parallel to the cathode-ray tube.

EXAMPLE 10

An optical film was obtained in the manner similar to that in Example 9,except that the rubber tacky layer had a refractive index of 1.505. Atransmission type liquid-crystal display device was obtained by usingthe thus obtained optical film.

EXAMPLE 11

An optical film was obtained in the manner similar to. that in Example9, except that the rubber tacky layer was replaced by an adhesive layermade of an acrylic ultraviolet-curing adhesive agent and having arefractive index of 1.52. A transmission type liquid-crystal displaydevice was obtained by using the thus obtained optical film.Incidentally, after brought in close contact with the liquid-crystaldisplay panel through the adhesive layer, the optical film wasirradiated with ultraviolet rays by a metal halide lamp so that theadhesive layer was cured to bond the optical film to the panel.

Comparative Example 10

A transmission type liquid-crystal display device was obtained in themanner similar to that in Example 9, except that the plurality ofoptical film having the optical path changing means was replaced by ascattering film. The scattering film was formed with a mold which had aroughened surface by sandblasting, so as to have substantially randomprismatic structures with a maximum inclination angle of about 15degrees measured by Talysurf made by TAYLOR-HOBSON Ltd.

Comparative Example 11

An optical film was obtained in the manner similar to that in Example 9,except that the rubber tacky layer was replaced by an acrylic tackylayer with a refractive index of 1.47. A transmission typeliquid-crystal display device was obtained by using the thus obtainedoptical film.

Evaluation Test 3

Frontal luminance in the center portion of the transmission typeliquid-crystal display device obtained in each of Examples 9 to 11 andComparative Examples 10 and 11 was measured by a luminance meter (BM-7made by TOPCON Corp.), which was located at a distance of 10 mm, 25 mmor 40 mm from the cathode-ray-tube-disposing side surface, in thecondition that the cathode-ray tube was switched on while theliquid-crystal cell was supplied with no voltage.

Results of the aforementioned measurement were shown in the followingTable 3.

Frontal luminance (cd/m²) Distance 10 mm 25 mm 40 mm Example 9 23 24 23Example 10 25 22 19 Example 11 24 25 25 Comparative Example 10  2  4  5Comparative Example 11 24 16 11

It is apparent from the Table 3 that frontal luminance in each ofExamples 9 to 11 superior to that in any one of Comparative Examples 10and 11 was attained and uniformity of luminance was also excellent. Inaddition, luminance and uniformity of luminance were higher in the orderof Comparative Example 11, Example 10, Example 9 and Example 11. Theresults correspond to the value of the refractive index of each adhesivemeans. Although variation of luminance was rarely recognized also inreal viewing in Examples 9 to 11, the display became darker in aposition farther from the light source in Comparative Example 11, andthe difference in luminance was visually recognized distinctly. Further,in Comparative Example 10, light was made to exit in a direction reverseto the light source at a large angle so that the exit light hardlycontributed to display. Therefore, the frontal luminance ran short tomake the display dark. It is proved from the above description thatdisplay which was bright and excellent in uniformity of brightness wasattained in Examples 9 to 11. Accordingly, according to the presentinvention, it is possible to form a transmission type orreflection-transmission double type liquid-crystal display device whichis prevented from increasing in volume and weight because of a lightpipe, and hence made small in thickness and light in weight by a filmsystem, and which is good in display quality.

Although the invention has been described in its preferred form with acertain degree of particularity, it is understood that the presentdisclosure of the preferred form can be changed in the details ofconstruction and in the combination and arrangement of parts withoutdeparting from the spirit and the scope of the invention as hereinafterclaimed.

1. An optical film comprising: a transparent film having an averagein-plane retardation not larger than 30 nm, wherein the transparent filmhas a thickness of 300 μm or less; an adhesive layer provided on onesurface of said transparent film, said adhesive layer having arefractive index different by 0.12 or less from a refractive index of alayer of said one surface of said transparent film; and a repetitiveprismatic structure provided on the other surface of said transparentfilm, said repetitive prismatic structure having optical path changingslopes aligned in a substantially constant direction at an inclinationangle in a range of from 35 to 48 degrees with respect to a plane ofsaid transparent film.
 2. An optical film according to claim 1, whereinsaid optical path changing slopes are constituted by at least two kindsof slopes in which one kind of slopes aligned in a substantiallyconstant direction serve as a reference while the other kind of slopesare aligned substantially in a direction which is opposite to said onekind of slopes; and wherein said adhesive layer is covered with a stripsheet.
 3. An optical film according to claim 1, wherein said transparentfilm has an average thicknesswise retardation of not larger than 50 nm.4. An optical film according to claim 1, wherein said transparent filmhas an average in-plane retardation of not larger than 20 nm and anaverage thicknesswise retardation of not larger than 30 nm.
 5. Anoptical film according to claim 1, wherein said inclination angle ofeach of said optical path changing slopes with respect to said filmplane is in a range of from 38 to 45 degrees.
 6. An optical filmaccording to claim 1, wherein said optical path changing slopes areformed based on a structure of grooves each shaped substantially like anisosceles triangle or any other triangle in section.
 7. An optical filmaccording to claim 1, wherein said optical path changing slopes areformed based on a structure of grooves or protrusions each shapedsubstantially like a tetragon or a pentagon m section.
 8. An opticalfilm according to claim 1, wherein a projected area, onto said filmplane, of flat surfaces each having an inclination angle of not largerthan 5 degrees with respect to said film plane is not smaller than 10times as large as a projected area, onto said film plane, of saidoptical path changing slopes.
 9. An optical him according to claim 1,wherein said prismatic structure includes optical path changing slopeseach having an inclination angle in a range of from 38 to 45 degreeswith respect to said film plane, and flat surfaces each having aninclination angle of not larger than 5 degrees with respect to said filmplane; wherein a projected width of each of said flat surfaces onto saidfilm plane is not smaller than 10 times as large as a projected width ofeach of said optical path changing slopes onto said film plane; andwherein said prismatic structure is formed into continuous grooves, eachof said continuous grooves being shaped substantially like a triangle insection, and each of said continuous grooves being extended from one endof said film to the other end thereof.
 10. An optical film according toclaim 1, wherein said prismatic structure having optical path changingslopes is formed into discontinuous grooves each shaped substantiallylike a polygon in cross-section; wherein a length of each of saiddiscontinuous grooves is not smaller than five times as large as a depthof each of said discontinuous grooves; wherein said optical pathchanging slopes are formed in a direction of the length of said groovesat an inclination angle in a range of from 38 to 45 degrees with respectto said film plane; and wherein a projected area of said discontinuousgrooves onto an area of said film plane is not larger than 10%.
 11. Anoptical film according to claim 1, wherein a reflection layer isdisposed closely on a surface of said film on which said prismaticstructure having said optical path changing slopes is formed.
 12. Anoptical film according to claim 1, wherein ridgelines of said opticalpath changing slopes are parallel to or inclined within an angle rangeof ±30 degrees with respect to one side of said transparent film.
 13. Anoptical film according to claim 1, wherein said adhesive layer is of alight diffusion type.
 14. An optical film according to claim 1, furthercomprising, in addition to said optical path changing slopes, steepslopes, each having an inclination angle of not smaller than 35 degreeswith respect to said film plane; wherein a projected area, onto saidfilm plane, of flat surfaces, each having an inclination angle of 5degrees or less with respect to said film plane, is greater than orequal to 10 times as large as a projected area, onto said film plane, ofsaid steep slopes, each having an inclination angle of not smaller than35 degrees with respect to said film plane.